A bimodal or a multimodal molecular weight distribution in certain resins indicates that the resin is comprised of at least two polymers of different molecular weights, more specifically, polymers of relatively higher and lower molecular weights. U.S. Pat. No. 4,461,873 found that the physical blending of different polymers having high and low molecular weights can lead to the formation of gel. Resins with biomodal or multimodal molecular weight distribution can also be produced using the melt blending technique described by Cozewith et al in WO 86/03756. However, such methods are expensive, cumbersome and time consuming.
Kuroda et al in U.S. Pat. No. 4,414,369, Kato et al in U.S. Pat. No. 4,420,592 and Raufast in U.S. Pat. No. 4,703,094 describe processes employing two reactors operating in series at different hydrogen concentrations to produce resin containing both high and low molecular weight polymer. In comparison to the bimodal resin produced by the physical blending of high molecular weight (HMW) and low molecular weight (LMW) resin, blends produced in these tandem processes were found to have an improved film appearance.
It is also known to use a single reactor containing blends of catalysts with different propagation and termination rate constants for ethylene polymerization for the production of broad or multi-modal molecular weight distribution polymer. U.S. Pat. No. 4,530,914 describes the use of a homogeneous catalyst system consisting of two different metallocenes for the production of broad molecular weight distribution and/or multi-modal molecular weight distribution polyolefin. U.S. Pat. Nos. 4,937,299 and 4,522,982 also describes homogeneous catalyst systems containing two or more metallocenes to produce blends of polymers with different molecular weights.
U.S. Pat. Nos. 4,701,432; 5,070,055; 5,032,562; 5,183,867; 5,525,678; 5,539,076; 5,614,456 and 5,882,750 describe silica-supported catalyst systems containing different active centers. The catalyst systems described in these patents are complicated and expensive, requiring for example expensive and troublesome aluminoxanes in their preparation. Further, silica supports which are suitable for these catalysts are prepared using spray drying or re-crystallization processes and high calcination temperatures which are complicated and/or expensive. Furthermore, the use of silica as a support results in the support remaining largely in the product, which can affect the product properties, such as optical properties, and processing.
One of the objects of the present invention is to overcome the above described problems with the prior art catalyst systems for producing bimodal and multimodal resins.